Frames to support eyeglasses on the face of the user probably developed soon after the first eyeglasses, at least several hundred years ago. Historically, they have been made of a large variety of materials, including bone, horn, metal, plastic etc. The making and marketing of eyeglasses is a worldwide trade involving hundreds of individual companies and totaling billions of dollars each year. The industry is stratified: certain companies make only components, others assemble components into frames, others are solely marketing. A very large component of customer satisfaction involves fashion. Eyeglass fashions change every few months.
An enhancement in satisfaction with the product can worth hundreds of millions of dollars per year. The introduction of prior art superelastic Nitinol eyeglass frames has led to growth of a highly competitive and litigious segment of the industry. Nitinol (also known as NiTi or TiNi) is an alloy of titanium nickel that undergoes an energetic crystalline phase change at near-ambient temperatures: these different phases have distinctly different mechanical characteristics giving rise to shape memory and superelasticity, which is the ability to recover more than 3-4 percent strain.
To date nearly all eyeglass frames have employed hinges to join the temple with the rim. Existing eyeglasses that do not have hinges and use ordinary material or superelastic nitinol are limited in the permitted flexure. These may suffer from stiffness, making them difficult to store in a compact space, and are subject to permanent distortion due to plastic deformation if elastic limit of the frame material is exceeded. To diminish this limitation, superelastic eyeglass frames and components have been known for more than a decade of years, and are a major selling item in eyeglass manufacturing and retail.
Superelastic SMA
Shape memory alloy materials (also termed SMA) are well known. One common SMA material is TiNi (also known as nitinol), which is an alloy of nearly equal atomic content of the elements Ti and Ni. Such an SMA material will undergo a crystalline phase transformation from martensite to austenite when heated through the material□s phase change temperature. When below that temperature the material can be plastically deformed from a “memory shape” responsive to stress. When heated through the transformation temperature, it reverts to the memory shape while exerting considerable force.
In the prior art many different useful devices employing SMA have been developed and commercialized. The typical SMAs used in the prior art devices are of polycrystalline form. Polycrystalline SMA exhibits both: 1) shape memory recovery (when cycled through the material's transformation temperature) and 2) superelasticity. The term superelasticity as used herein applies to a polycrystal SMA material which, when above the transformation temperature (in the austenite crystalline phase), exhibits a strain recovery of several percent. This is in comparison to a strain recovery on the order of only about 0.5 percent for non-SMA metals and metal alloys. Polycrystalline alloys, including Nitinol, cannot achieve the maximum theoretical strain recovery because not all of the crystal grains are optimally aligned.
Superelasticity in a polycrystal SMA material results from stress-induced conversion from austenite to martensite as stress is increased beyond a critical level, and reversion from martensite to austenite as stress is reduced below a second (lower) critical level. These phenomena produce a pair of plateaus of constant stress in the plot of stress versus strain at a particular temperature. Single crystal superelasticity is characterized by an abrupt change in slope of the stress strain plot at a combination of stress, strain, and temperature characteristic of that particular alloy.
Hyperelastic SMA
Shape memory copper-aluminum based alloys grown as single crystals have been experimentally made in laboratories, typically in combination with about 5 percent Ni, Fe, Co, or Mn. The most common such CuAl-based alloy is CuAlNi, which is used throughout this description as the primary example; others are CuAlFe, CuAlCo, and CuAlMn. Single crystal SMA materials when stressed have the property of enabling a shape memory strain recovery much greater than polycrystalline SMA, and resulting shape recovery from about 9% to as great as 24% when above the phase change transition temperature. Because such strain recovery is so far beyond the maximum strain recovery of both convention polycrystal SMA materials and non-SMA metals and alloys, the strain recovery property of single crystal SMA will be referred to herein as “hyperelastic.”